Converters that are based on pulse width modulation schemes are equipped with switches for connecting converter AC outputs, which are typically connected to an AC equipment or a grid for power transmission, in an alternating fashion to a high or low voltage level of a DC link through DC converter inputs. The switching is performed on the basis of a pulse width modulation scheme which provides the timing for connecting the AC outputs to and disconnecting the AC outputs from the voltage levels of the DC link. During the time an AC output is connected to the DC link a current flows from the link to the output, or vice versa.
Inverters are often used to connect wind turbines to a grid. Wind turbines convert wind energy to electrical energy by using the wind to drive the rotor of a generator, either directly or by means of a gear box. The AC frequency that is developed at the stator terminals of the generator, often called “stator voltage” is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as function of speed and, depending on the particular type of generator, on the flux level. For optimum energy capture, the speed of rotation of the output shaft of the wind turbine will vary according to the speed of the wind driving the wind turbine blades. To limit the energy capture at high wind speed, the speed of rotation of the output shaft is controlled by altering the pitch of the turbine blades. Matching of the variable voltage and frequency provided by the generator to the nominally constant voltage and frequency of the grid can be achieved by using a power converter.
In a first stage of a power converter a rectifier is used to convert the AC voltage delivered from the generator to a DC voltage with high voltage level and a DC voltage with a low voltage level. These DC voltages are fed to a so called DC link as a high voltage level of the link and a low voltage level of the link. In a second stage, an inverter which is connected to the DC link uses switches driven by a pulse width modulation scheme, as mentioned above, to convert the DC voltage to an AC voltage matching the voltage level, the grid frequency and the power factor requested by the grid operator. Instead of the power factor, which is given by the ratio of the real power (P) to the apparent power (S) (which is the sum of the squares of the real power and the reactive power) the inverter can also be controlled on the basis of a real power demand and a reactive power demand. Furthermore, instead of controlling the inverter directly according to the power factor, or the real and reactive power, it can also be controlled by current demand signals since the voltage amplitude is usually a fixed parameter in the grid so that the power fed to the grid by the inverter can be defined by current amplitudes and phase angles between the current and the voltage. Hence, a power factor demand signal or demand signals for active and reactive power can be converted to current demand signals which are then used for controlling the inverter, i.e. for determining the timing of pulses causing the switches to open and close. Such mode of control is known as current control.
A typical power converter including an active rectifier and an active inverter for converting the AC power of a wind turbine generator to DC power and the DC power again to AC power is, for example, described in US 2009/0147549 A1.
An inverter converting a DC voltage into AC voltage by use of a so called sliding mode control, in which the inverter is controlled as a result of comparison of an output state with a reference quantity and a switching law which is dependent on a state error, is disclosed in U.S. Pat. No. 5,388,041.
US 2007/0216373 A1 discloses three phase controllers for power converters having a control core which can be implemented using different control techniques, in particular one cycle control, which is a special pulse width modulation control method, and sliding mode control.
With a grid connected inverter, for example of a wind turbine, any sudden changes in grid voltage such as those which occur during a grid fault have to be accompanied by a matched rapid change of inverter terminal voltage, in order to limit the size of the inverter current transient and to allow the inverter to maintain current control and stay connected to the grid. Typically, the time in which the inverter output voltage has to be changed to adapted to the change in grid voltage is much shorter than the update time of the control system determining output voltage state. Failure to modify the actual inverter output voltage rapidly in these conditions leads to very high current flowing in the inverter module which may be above the instantaneous over-current threshold for the inverter module which then may lead to a trip condition of the inverter and, hence, of the wind turbine, which is to be avoided since it is often a requirement to stay connected to the grid during such a grid fault.
Additional features are therefore required to maintain the output current below instantaneous over-current levels with a time constant appropriate to the circuit parameters, i.e. DC link voltage, network impedances, network filter impedances, power semi-conductor device over current protection thresholds, etc. Furthermore, features are required in the controls to re-establish control of the current in a dynamic and robust manner.
The issue of protecting a power semi-conductor device from tripping or becoming stressed has previously been addressed as follows:
Typically, existing schemes add a fast acting current limiting block after the normal current control function. This current limiting block is based on fixed positive and negative thresholds below/above which the output current is not permitted. When an output current of a phase is above the current limiting threshold, or below, in case the instantaneous current is negative, the power semi-conductor device managing that current is turned off, so limiting further increase in current. The power semi-conductor device is turned back on against some lower threshold condition being achieved, or a time delay. These thresholds are set at some margin below the instantaneous over-current thresholds.
The issue of maintaining robust control of the current from the inverter to the grid has previously been addressed as follows: In a conventional vector controlled inverter, the current controller requires additional gain during such a grid voltage transient. This is achieved by means of a grid voltage feed-forward term in the current controller. In the steady state, this feed-forward term is heavily filtered to maximise stability. During the transient at the start and the end of a grid fault, this feed-forward term is very lightly filtered. This light filtering is a compromise between dynamic performance and grid stability.
Current limiting achieved by a dual mode control of a pulse width modulation motor drive is described in U.S. Pat. No. 4,904,919. In this dual mode control, a method and a control circuit for switching between sine-triangle modulation and hysteresis modulation in a pulse width modulation drive are used to overcome difficulties which arise in sine-triangle modulation during transient conditions which may cause large undesirable instantaneous values for the current in the pulse which modulated generated wave. To prevent from such undesirable instantaneous values for the current a sine-triangle pulse width modulation scheme is overridden with a hysteresis modulation scheme in which the phase voltage command is controlled in response to the magnitude of a sensed phase current reaching a hysteresis band upper limit so as to produce a limit current represented by the threshold. At a later time there is a reversion to sine-triangle modulation in response to the magnitude of the sensed phase current decreasing to a hysteresis band lower limit.